Nuclear reactors are located near bodies of water, introducing unique flood-related risks.

Nuclear power plants are always situated near a body of water—a river, lake, estuary or ocean—because they require a plentiful, reliable source of water for cooling purposes. In the absence of cooling water, a nuclear reactor will overheat, leading to core damage, containment failure, and release of harmful radiation into the environment.

However, water can quickly turn dangerous when floods occur. Flooding can damage equipment or knock out the plant’s electrical systems, disabling its cooling mechanisms. This is what happened at the Fukushima Dai-ichi plant in Japan as a result of the March 2011 tsunami, causing severe damage to several of the plant’s reactors.

Floods due to natural causes

While tsunamis are not a significant risk for most U.S. nuclear power plants, there are other natural weather events that can lead to flooding. Heavy rain or snow can cause rivers to overflow, and tropical storms or nor’easters can cause storm surges that threaten coastal plants.

Floods from such natural weather events have caused problems at several U.S. nuclear power plants in recent years. In June 2011, unusually high water on the Missouri River, caused by a combination of heavy spring rains and Rocky Mountain snowmelt, inundated the Fort Calhoun plant in Nebraska. And in October 2012, flooding from Hurricane Sandy caused two New Jersey nuclear plants, Salem and Oyster Creek, to shut down when high water levels threatened their water intake and circulation systems.

Floods caused by dam failures

Not all floods that threaten nuclear reactors have natural causes, however. Many nuclear plants are situated near rivers, and some of them are downstream from a dam. When the dam fails, the resulting flood is sudden and can be catastrophic. Unlike river overflows or hurricanes, dam failures are likely to occur with little or no advance warning, leaving plant operators scrambling to protect their facilities before the floodwaters arrive, typically within hours.

So far, dam failures have not affected any U.S. nuclear power plants. But in July 2011 we learned that we may been luckier than we knew, as the NRC released a report stating that previous estimates of flood risk for many reactors were based on outdated information and would need to be revised upward.

As the 2011 NRC report points out, dam failures are far from rare; there have been more than 700 of them in the U.S. since 1975. The NRC has estimated the likelihood of failure at one particular dam—the Jocassee dam, which lies a few miles upstream of the Oconee Nuclear Station in South Carolina—at approximately 1 in 180 over the 20 years remaining on its license to operate.

While 1 in 180 may sound like a reasonably low probability, it is high enough to require corrective action according to NRC standards. And when we consider that 34 nuclear plants lie downstream from more than 50 dams, the cumulative likelihood of at least one plant being affected by a dam failure is too high to ignore—especially since these risk estimates do not account for the impacts of earthquakes or the possibility of sabotage.

The NRC’s responsibility

Almost as worrisome as the threat of dam failure itself is the fact that the NRC apparently was aware of the increased risk for years before addressing it—and passages indicating this were blacked out in the 2011 report on its original release, according to an NRC engineer, Richard Perkins, who contacted the agency’s Inspector General in September 2012. The NRC had claimed that the redactions were necessary for security reasons, but Perkins asserted that the agency’s real motive was to avoid embarrassment.

The NRC should fulfill its responsibility to the public and act to ensure that the threat of flood risk is adequately addressed at our nation’s nuclear plants.

Nuclear Time Bomb: Flood Risk from Dam Failure

The following 34 U.S. nuclear plants were identified in the NRC’s 2011 report as being at heightened risk of flood damage due to upstream dam failures.